Flux-switching-transformer head shaft encoder



A ril 28, 1964 WQLFF 3,131,387

FLUX-SW1TCHING-TRANSFORMER HEAD SHAFT ENCODER Filed June 21, 1963 FIG.|

FIG.2

INVENTOR.

United States Patent 3,131,387 FLUX-WETQHHNG-TRANSFURMER HEAD SHAFT ENCODER Gunther Woitf, Southport, Conan, assignor to Machine Tool Automation, inc, Southport, Conn, a corporation of Connecticut Filed June 21, 1963, Ser. No. 289,563 1 Ciaim. (Cl. Seth-347) This invention relates to apparatus capable of transducing a mechanical positional quantity into a quantized and/ or coded electrical signal, such apparatus commonly being referred to as a shaft encoder. More specifically, this invention relates to a new and improved shaft encoder in which a high ratio of voltages, corresponding to two distinct states of the code member, is achieved, thus permitting the attainment of a greatly improved Yes-No ratio.

U.S. Patent No. 2,942,252 discloses a shaft encoder in which the magnetic reluctance-to-electrical voltage transducing element consists of two coils wound on a magnetic circuit comprised of a horseshoe-shaped stationary portion and a movable armature portion. This arrangement is a mechanically simple one, and results in a transducer with outputs quite acceptable for many applications, particularly if a constant current source of power is employed. However, as computers become increasingly more sophisticated, ever greater demands are placed on the signal-noice ratios of the inputs and also a requirement for constant current excitation becomes burdensome.

In the present art, the output voltage is directly proportional to the number of flux linkages because both N and dt are essentially constant. Ideally, therefore, the value of p should alternate between zero and some finite value. However, the voltage applied across the exciting coil must, at least in part, be opposed by this same 1p, viz.,

Thus, it is seen that as the reluctance of the magnetic path is increased and the flux decreases, either e must decrease (as in a constant current generator), or the current i must increase. But ocN i, so that with increased exciting current the flux will tend to increase. Therefore, an increase in the reluctance of the original flux path results in a less than proportional decrease of secondary flux due to an increase in primary current (for a non-constant current generator) and due to flux lines seeking other available flux paths. The above shows that for maximum secondary winding response to a change in the reluctance of the flux path through the code member, the total primary flux should be maintained at a constant value, and this the prior art does not achieve.

It is therefore an object of this invention to provide a transducing head for a shaft encoder which will yield high Yes-No signal ratios and still be of a simple and inexpensively produced design.

It is a further object of this invention to provide a transducing head for a shaft encoder which will be capable of producing high Yes-No signal ratios even when excited from other than a constant current source.

These objects of the invention are achieved by providing a flux-switching magnetic structure, which varies with the amount of flux linking with the secondary coil, and maintains constant flux in the primary coil.

The above, as well as other objects and novel features of the invention will become apparent from the follow ing specification and accompanying drawings in which:

FIG. 1 is a sectional elevational view of a flux-switching-transformer head shaft encoder to which some of the principles of the invention have been applied; and

FIG. 2 is a sectional stretch-out, taken along line 2-2 of FIG. 1.

Referring to the drawings, and particularly to FIG. 1, some of the principles of the invention are shown as applied to a shaft encoder, including a codified member it) and a fiux-switching-transformer head 11. Relative motion between codified member 10 and head 11 results whenever the input to the encoder (angle of the shaft which is not shown) is varied. In the particular configuration shown, the code member and head are round, suitable for encoding an angular input. It is, of course, within the scope of this invention to form the code member and head in a flat fashion, thus permitting the encoding of linear displacements; or to form the code member in the shape of a flat disc instead of a drum shape as shown.

The head 11 consists of a plurality of magnetic laminations 12, an exciting or primary coil 13, and a pick-off or secondary coil 14. As a voltage is impressed across coil 13, an M.M.F. exists in the center leg 15 of the magnetic circuit. This in turn will produce fluxes in the remaining magnetic material as well as in the air space surrounding it in accordance with the relative permeabilities of the media. The primary current, and therefore the flux, will increase until enough flux has been generated to oppose the applied voltage less the IR drop. It will be seen that most of the flux will pass through leg 16 because the undulating pattern of the codified member presents a short air gap to said leg 16. Conversely, very little flux will pass through leg 17, and therefore only a very small output voltage will be generated from coil 14.

If now the input shaft is rotated, no change in the output will occur until the air gap conditions are changed as a result of the undulating pattern on the inside of the drum. As soon as a raised portion leaves leg 16, a corresponding raised portion appears opposite leg 17, and a high output voltage is generated. Note, however, that the flux linking with the primary coil remains essentially independent of rotation of the input shaft.

It will be apparent to those skilled in the art that various modifications of the above arrangement are possible. For instance, the center leg 15 may be rotated degrees, as the effect of its air gap on the total reluctance can be made very small. Furthermore, it is possible to reduce the effect of the unwanted flux through the large air gap leg by adding an opposing pick-off coil to it, thus subtracting out unwanted leakage flux; or said second pickotf coil may be used to generate the complement of the digital number being read. Also, the pick-off leg may be constructed in a bifurcated fashion, with two coils on each projection, to assist in ambiguity resolution.

What is claimed is:

In a device capable of transducing a mechanical positional quantity into a quantized and/or coded electrical signal, a pole surrounded by an exciting coil, another pole surrounded by a pick-off coil, and a third pole so arranged that as the flux linking with the exciting coil tends to diminish due to the increased reluctance of said pick-off coil pole, said third pole provides additional permeability to compensate for loss of permeability in said pick-01f coil pole, and thus maintains the exciting coil pole flux constant.

References Cited in the file of this patent UNITED STATES PATENTS 3, ,145 Wolff Oct. 3, 1961 

